Adiponitrile (ADN) is a commercially important and versatile intermediate in the industrial production of nylon polyamides useful in forming films, fibers, and molded articles. ADN can be produced by hydrocyanation of 1,3-butadiene (BD) in the presence of transition metal complexes including various phosphorus-containing ligands. For example, catalysts including nickel and monodentate phosphorus-containing ligands are well documented in the prior art; see, for example, U.S. Pat. Nos. 3,496,215; 3,631,191; 3,655,723 and 3,766,237; and Tolman, C. A., McKinney, R. J., Seidel, W. C., Druliner, J. D., and Stevens, W. R., Advances in Catalysis, 1985, Vol. 33, pages 1-46. Improvements in the hydrocyanation of ethylenically unsaturated compounds with catalysts including nickel and certain multidentate phosphite ligands are also disclosed; e.g., see: U.S. Pat. Nos. 5,512,696; 5,821,378; 5,959,135; 5,981,772; 6,020,516; 6,127,567; and 6,812,352.
The hydrocyanation of activated olefins such as conjugated olefins (e.g., 1,3-butadiene) can proceed at useful rates without the use of a Lewis acid promoter. However, the hydrocyanation of un-activated olefins, such as 3-pentenenitrile (3PN), require at least one Lewis acid promoter to obtain industrially useful rates and yields for the production of linear nitriles, such as ADN. For example, U.S. Pat. Nos. 3,496,217, 4,874,884, and 5,688,986 disclose the use of Lewis acid promoters for the hydrocyanation of non-conjugated ethylenically unsaturated compounds with nickel catalysts including phosphorus-containing ligands. As a result, in a two-step conversion of BD to ADN, the first hydrocyanation step converting BD to 3PN can be carried out in the absence of a Lewis acid promoter, while the second hydrocyanation step converting 3PN to ADN is facilitated by use of a Lewis acid, e.g., ZnCl2. Typically, such reactions have been run in as complete an absence of water as is practicable, e.g., to avoid phosphite ligand hydrolysis.
As is disclosed in a copending application by Applicants herein, a process for hydrocyanation of 3-pentenenitrile can include feeding 3-pentenenitrile and HCN to a hydrocyanation reaction zone including a Lewis acid promoter, nickel and a phosphorus-containing ligand, in the presence of water, preferably a controlled concentration of water. In that application, incorporated by reference herein in its entirety, the unexpected discovery was disclosed that by maintaining particular concentrations of water in the reaction mixture as the catalyst is recycled through a catalyst recovery process, the hydrocyanation catalyst inventory maintains its activity through a greater number of recycle cycles than other processes that have water at higher or lower, i.e., near zero, concentrations. It is disclosed therein that it was found to be advantageous for various reasons described therein to maintain a range of water concentrations within the reaction zone sufficient to improve activity of the catalyst inventory under continuous operation with downstream liquid-liquid extraction and recycle of the catalyst complex.
However, the presence of water in the hydrocyanation reaction zone, while improving activity of the catalyst inventory, can lead to hydrolysis of certain triarylphosphite ligands of the nickel catalyst to yield acidic phosphite ligand hydrolysis products (LHP), such as diarylphosphites produced by hydrolysis of triarylphosphites, that can undergo further degradation to monoarylphosphites and phosphorous acid. Insomuch as the ligand hydrolysis process itself can be acid-catalyzed, the buildup of acidic LHP can further increase the rate of ligand hydrolysis, degrading the active nickel-phosphite hydrocyanation catalyst.